I have a Scag Turf Tiger ZTR with the 27 h.p. Kohler Command series CH740 engine that has died and defies troubleshooting. The grass is growing and I am desperate for suggestions.

Quick summary: The spark plugs fire with the ignition switch in the start position, but stop firing as soon as the switch goes to the run position.

Details:
The Scag has 560 hours and has been completely reliable up until now. I had just started mowing, and about 5 minutes after having started the engine the engine backfired very loudly and immediately died. (I had just started a right turn after having mowed a straight line for about 2 minutes when it happened, probably just a coincidence, but it did happen at the instant I started the turn.) When I looked at the engine the air hose from the air cleaner was off the carburetor, presumably blown off by the force of the backfire.

When I crank the engine it fires and sounds as if it is going to run, but when I release the key from the start position the engine it dies immediately. I have used a spark plug tester that shows both cylinders are sparking with the ignition in the start position, and the spark stops instantaneously on putting the switch in the run position.

I have confirmed fuel is being pumped and the carburetor fuel shutoff solenoid is operating correctly. I replaced the spark plugs. When I pull plugs right after attempting to start the engine they are damp and have a gas odor, consistent with the engine turning over a few turns after it lost spark. Visual inspection of the carburetor shows nothing amiss, linkages are working correctly, although the throttle valve moves little from the full open position, which is where I normally run it. I can see fuel pulse into the carburetor throat when I look down it with the air intake hose disconnected while cranking the engine. It appears that the fuel supply is not the cause. To confirm there was no problem with the gas quality I drew gas directly from a jug with my freshest gas with a good dose of Sea Foam fuel treatment; there was no change.

I have checked that there are no obstructions to the air flow and attempted starting with the air cleaner removed. I find no problem with the air flow.

The local Scag guy said he has seen similar symptoms with a broken valve push rod. I pulled the valve covers and all push rods appear sound and straight. Pushing on the rocker arms showed all valves moving freely. Compression is good--143 psi in both cylinders.

I disconnected the kill circuit wire where the Scag wiring meets the Kohler wiring, it made no difference.

When I disconnected the kill circuit wires directly from the ignition modules I had no spark at all, even with the ignition in the start position.

I checked out the ignition switch as best I could. There were no obvious defects, but a new one did not cost much so I replaced it. That made no difference.

One of the possible causes listed in the Kohler manual is a bad diode. My multimeter reported a bad diode which is incorporated in a relay. I replaced the relay/diode. It made no difference.

I have checked wiring connections. All seem okay, there is no apparent damage to the wiring harness.

There is an electronic module that does some magic involving safety switches and ignition. I can find no information on testing it, and it is too expensive to replace just on the off chance it has gone bad.

I am well and truly stumped, and desperate for suggestions. In addition to suggestions for troubleshooting and repair, I would welcome information how to jury rig it so it might run well enough to do some desperately needed mowing pending a full repair. The local professional mechanics are backed up 2 to 3 weeks on repairs, and I cannot wait for them.